Bright nickel electroplating baths are used in the automotive, electrical, appliance, hardware and various other industries. One of the most commonly known and used nickel electroplating baths is the Watts bath. A typical Watts bath includes nickel sulfate, nickel chloride and boric acid. The Watts bath typically operates at a pH range of 2-5.2, a plating temperature range of 30-70° C. and a current density range of 1-6 amperes/dm2. Nickel sulfate is included in the baths in comparatively large amounts to provide the desired nickel ion concentrations. Nickel chloride improves anode corrosion and increases conductivity. Boric acid is used as a weak buffer to maintain the pH of the bath. In order to achieve bright and lustrous deposits, organic and inorganic brightening agents are often added to the baths.
A common problem with most metal plating baths is recovery of the bath components and disposal of break-down products after use. While some bath components may be readily recovered, although recovery processes may be costly, other components and break-down products may be difficult to recover and are discharged in waste water, thus potentially contaminating the environment. In the case of the Watts bath, nickel sulfate and nickel chloride may be readily recovered; however, recovery of boric acid is challenging and often ends up in waste water contaminating the environment.
Many governments around the world are passing stricter environmental laws and regulations with respect to how chemical waste is treated and the types of chemicals industries may use in development and manufacturing processes. For example, in the European Union the regulation Registration, Evaluation, Authorization and Restriction of Chemicals, known as REACh, has banned numerous chemicals or is in the process of banning chemicals such as boric acid from substantial industrial use. Accordingly, the metal plating industries which manufacture and sell electroplating baths which typically include boric acid have attempted to develop boric acid free baths. In the case of nickel electroplating baths, many manufacturers have tried to address the problem of developing a nickel electroplating bath free of boric acid with substantially the same plating performance by substituting the boric acid with nickel acetate. Unfortunately, nickel acetate baths often produce rough and insufficiently dense nickel deposits which vary in appearance depending on the current density applied. In addition, depending on the amount included in the nickel baths, nickel acetate based baths may generate an offensive odor, thus compromising the working environment.
Another compound typically included in nickel electroplating baths to improve plating performance which is now frowned upon by the governments of many countries is coumarin Coumarin has been included in nickel plating baths to provide a high-leveling, ductile, semi-bright and sulfur-free nickel deposits from a Watts bath. Leveling refers to the ability of the nickel deposit to fill in and smooth out surface defects such as scratches and polish lines. An example of a typical nickel plating bath with coumarin contains about 150-200 mg/L coumarin and about 30 mg/L formaldehyde. A high concentration of coumarin in the bath provides very good leveling performance; however, such performance is short-lived. Such high coumarin concentrations result in a high rate of detrimental breakdown products. The breakdown products are undesirable because they can cause non-uniform, dull gray areas in the deposit that are not easily brightened by subsequent bright nickel deposits. They can reduce the leveling performance of the nickel bath as well as reduce other beneficial physical properties of the nickel deposit. To address the problem workers in the industry have proposed to reduce the coumarin concentrations and add formaldehyde and chloral hydrate; however, use of such additives in moderate concentrations not only increases tensile stress of the nickel deposits but also compromise leveling performance of the baths. Further, formaldehyde, as boric acid and coumarin, is another compound which many government regulations, such as REACh, consider harmful to the environment.
It is important to provide highly leveled nickel deposits without sacrificing deposit ductility and internal stress. The internal stress of the plated nickel deposit can be compressive stress or tensile stress. Compressive stress is where the deposit expands to relieve the stress. In contrast, tensile stress is where the deposit contracts. Highly compressed deposits can result in blisters, warping or cause the deposit to separate from the substrate, while deposits with high tensile stress can also cause warping in addition to cracking and reduction in fatigue strength.
As briefly mentioned above, nickel electroplating baths are used in a variety of industries. Nickel electroplating baths are typically used in electroplating nickel layers on electrical connectors and leadframes. Such articles have irregular shapes and are composed of metal such as copper and copper alloys with relatively rough surfaces. Therefore, during nickel electroplating, the current density is non-uniform across the articles often resulting in nickel deposits which are unacceptably non-uniform in thickness and appearance across the articles.
Another important function of nickel electroplating baths is to provide a nickel underlayer for gold and gold alloy deposits to prevent the corrosion of underlying metals plated with gold and gold alloy. Prevention of gold and gold alloy pore formation which leads to corrosion of underlying metals is a challenging problem. The pore formation of gold and gold alloy plated articles has been especially problematic in the electronic materials industry where corrosion can lead to faulty electrical contacts between components in electronic devices. In electronics gold and gold alloys are used as solderable and corrosion resistant surfaces for contacts and connectors. Gold and gold alloy layers are also used in lead finishes for integrated circuit (IC) fabrication. However, certain physical properties of gold, such as its relative porosity, translate into problems when gold is deposited on a substrate. For instance, gold's porosity can create interstices on the plated surface. These small spaces can contribute to corrosion or actually accelerate corrosion through the galvanic coupling of the gold layer with the underlying base metal layer. This is believed to be due to the base metal substrate and any accompanying underlying metal layers which may be exposed to corrosive elements via the pores in the gold outer surface.
In addition, many applications include thermal exposure of coated leadframes. Diffusion of metal between layers under thermal aging conditions may cause a loss of surface quality if an underlying metal diffuses into a noble metal surface layer.
At least three different approaches of overcoming the corrosion problems have been attempted: 1) reducing the porosity of the coating, 2) inhibiting the galvanic effects caused by the electropotential differences of different metals, and 3) sealing the pores in the electroplated layer. Reducing the porosity has been studied extensively. Pulse plating of the gold and utilization of various wetting/grain refining agents in the gold plating bath affect the gold structure and are two factors contributing to a reduction in gold porosity. Often regular carbon bath treatments and good filtration practices in the series of electroplating baths or tanks combined with a preventive maintenance program help to maintain gold metal deposition levels and correspondingly low levels of surface porosity. A certain degree of porosity, however, continues to remain.
Pore closure, sealing and other corrosion inhibition methods have been tried but with limited success. Potential mechanisms using organic precipitates having corrosion inhibitive effects are known in the art. Many of these compounds were typically soluble in organic solvents and were deemed not to provide long term corrosion protection. Other methods of pore sealing or pore blocking are based on the formation of insoluble compounds inside pores.
In addition to the problem of pore formation, exposing gold to elevated temperatures, such as in thermal aging, undesirably increases the gold's contact resistance. This increase in contact resistance compromises the performance of the gold as a conductor of current. In theory, workers believe that this problem arises from the diffusion of organic materials co-deposited with the gold to the contact surfaces. Various techniques for obviating this problem have been attempted heretofore, typically involving electrolytic polishing. However, none have proven completely satisfactory for this purpose and investigative efforts continue.
Accordingly, there is a need for nickel electroplating compositions and methods to provide bright and uniform nickel deposits, even across a wide current density range, good ductility and which can be used as underlayers to reduce or inhibit pitting and pore formation in gold and gold alloy layers, thus preventing corrosion of underlying metal.